1. Field of the Invention
The present invention relates to an image scanning apparatus and method. More particularly, the present invention relates to an image scanning apparatus and method in which shading data of the image scanning apparatus is acquired when manufacturing the image scanning apparatus, which is when a white reference or the shading data is cleanest, and an original document is scanned using the shading data, thereby eliminating distortion due to white lines in an image caused by pollutants attached to the shading reference or the white reference and improving the performance of the image scanning apparatus.
2. Description of the Related Art
An image scanning apparatus such as a scanner scans printed materials, photographs, or characters/pictures written/drawn by hand into color or black and white images. Image scanning apparatuses are employed in multifunction printers, translators, computers for computer aided design (CAD), facsimiles, character recognizers, and digital copiers, and are divided into sheet-fed systems and flatbed scanning systems. A sheet-fed system such as a facsimile reads a page of an original document, which moves while an image sensor included in the sheet-fed system is stationary. When an automatic document feeder (ADF) is added to the sheet-fed system, multiple pages of the original document can be automatically scanned. On the other hand, a flatbed system such as a copier can scan books as well as a page of an original document. In the flatbed system, the original document is stationary while an image sensor included in the flatbed system moves.
An image sensor such as a charge coupled device (CCD) or a contact image sensor (CIS) produces an electrical signal proportional to the amount of light reflected by the original document. Hence, a pixel pattern corresponding to an original image is generated. An image scanning apparatus including a CCD uses a halogen lamp or a fluorescent lamp as a light source while an image scanning apparatus including a CIS uses a small light emitting diode (LED) as a light source. A black and white CIS reads black and white on an original document using only a green LED. On the other hand, a color CIS detects the amount of each of red, green, and blue (RGB) light beams alternately irradiated from RGB LEDs and reflected from an original document, and recognizes the color of the original document by combining the amount of detected light of each of the RGB light beams.
The amount of light emitted from these small LEDs is very small compared with external light, let alone light emitted from a light source such as a lamp. Therefore, it is very important to prevent the CIS from being exposed to the external light because, when the external light is mixed with the light reflected by the original document, the amount of incident light increases and, accordingly, the CIS detects an image whiter than the original document.
A sheet-fed type image scanning apparatus can easily block the external light since it is not exposed to the external light. However, a flatbed type image scanning apparatus can be exposed to the external light when its scanning cover is open. Therefore, special care is needed to prevent the flatbed image scanning apparatus from being exposed to the external light.
Meanwhile, if the light reflected by an original document is not directly incident on the image sensor or if a deviation occurs due to a tolerance when assembling an optical system, the image sensor fails to measure the exact amount of light reflected by the original document. As a result, an image may be distorted. This is called a shading phenomenon. Generally, an image scanning apparatus corrects shading before scanning the original document such that data can be read at a stable level.
“Shading correction” is a process in which, for example, a white level is adjusted by dummy scanning a shading reference such as a white panel, a white roller, a white bar, or a white sheet before scanning an original document to compensate for shading in which non-uniform luminosity is read. For the shading correction, the shading reference is interposed between a home position of the image sensor and a scanning area.
Shading correction will now be described in more detail with reference to FIG. 1. FIG. 1 is a block diagram of a conventional multifunction printer (MFP) including an image scanning apparatus. The multifunction printer performs the functions of a printer, a copier, a scanner, and a facsimile. Referring to FIG. 1, a central processing unit (CPU) 10 controls the entire operation of the MFP and a system memory 60 including a program driving the MFP. The CPU 10 exchanges key input data and data that can be displayed on a display 42 with an operation panel (OPE) 40 including a microcomputer via a universal asynchronous receiver transmitter (UART) channel using serial communications. Typically, the display 42 is a liquid crystal display (LCD). The CPU 10 controls an image processing unit 20 for processing an image to be scanned by a scanner module 30. The CPU 10 controls a modem 90 for transmitting/receiving data to/from an external device via a public switched telephone network (PSTN). The CPU 10 also controls a universal serial bus (USB) 72 for transmitting/receiving image data to/from a personal computer (PC).
A printer drive unit 74 controls printing when print data from the PC or fax data is received. The image processing unit 20 performs shading/gamma corrections, enlargement/reduction conversions, edge emphasis, or error diffusion on image data received from the scanner module 30. The scanner module 30 scans an original document. The scanner module 30 includes RGB LEDs as an image sensor as well as a light source, and a lens.
The OPE 40 can include, for example, 3×4 telephone button keys and other functional keys. The display 42 displays the current status of the multifunction printer. An image memory 50 stores the image data to be processed by the image processing unit 20. The system memory 60 includes a flash memory storing entire programs of the multifunction printer and a volatile synchronous dynamic random access memory (SDRAM) for processing other data. A printer 80 prints data received from the printer drive unit 74.
FIG. 2 is a block diagram of the image processing unit 20 of the conventional image scanning apparatus of FIG. 1. Referring to FIG. 2, the image processing unit 20 includes an analog/digital (A/D) converter 22, a scanner controller 24, and an image processor 26. The A/D converter 22 converts analog image data received from the scanner module 30 into digital image data. The scanner controller 24 includes a data transmission module 24a and generates a master clock signal, a shift register clock signal, and a transmission gate clock signal.
The MFP in such a structure corrects shading as follows. Light emitted from an LED of the scanner module 30 is reflected by a shading reference such as a white panel, a white roller, a white bar, or a white sheet, and incident on an image sensor. The image sensor generates a voltage proportional to the amount of the incident light. The A/D converter 22 of the image processing unit 20 converts analog image data output from the image sensor into digital image data. The digital image data of the shading reference, i.e., luminosity, should be maintained at a certain level.
However, when the light reflected by the shading reference is not precisely incident on the image sensor, the voltage output from the image sensor decreases. Accordingly, the white level is reduced, thereby causing shading of an image.
In a conventional shading correction method, a shading correction factor corresponding to the reduced white level is calculated such that the value of luminosity obtained by scanning the shading reference is maintained at a predetermined value. The detected shading correction factor is applied to each pixel of data scanned from the original document. Shading data for correcting the reduced white level is added to the scanned data, thereby correcting the distorted image.
In this method, when scanning the original document, shading is compensated for in consideration of a case where the light emitted from the LED is not directly incident on the image sensor or where external other light is incident on the image sensor. Therefore, this method has been effective to a certain degree.
Another conventional method similar to this shading correction method is disclosed in U.S. Pat. No. 6,421,146, which is incorporated herein by reference.
In the previous conventional method, however, pollutants such as paper dust or foreign matter attached to the shading reference cannot be properly compensated for. Typically, a shading reference of an image scanning apparatus is exposed to paper dust or foreign matter that enters the image scanning apparatus when scanning an original document. Therefore, when white shading data is acquired based on the shading reference while the pollutants are attached to the shading reference, shading correction is performed on the basis of the pollutants. Therefore, when scanning the original image, image quality may be degraded.
This problem is serious for a sheet-fed image scanning apparatus using a white reference exposed externally as a shading reference. In other words, the sheet-fed image scanning apparatus acquires shading data before scanning an original document. However, if the shading reference is contaminated with paper dust or foreign matter, the sheet-fed image scanning apparatus scans the contaminated shading reference and performs shading correction based on the scanning result, thereby degrading image quality.